I think people are over complicating the answer here. If you want a series of radio buttons lined up simply wrap your inputs in label elements:
<div class="form-field">
<label><input type="radio" name="lump">Lump</label>
<label><input type="radio" name="stump">Stump</label>
<label><input type="radio" name="bump">Bump</label>
</div>
This will line the radio buttons one next to the other. If you want them running straight up and down in your CSS add display:block to your label elements. This works down to IE7. And here's the relevant section of the HTML spec: http://dev.w3.org/html5/spec/single-page.html#the-label-element.
So for your code, this should work:
<div class="form-field">
@Html.Label("Gender")
<label>Male @Html.RadioButton("gender", "Male")</label>
<label>Female @Html.RadioButton("gender", "Female")</label>
<label>Unkown @Html.RadioButton("gender", "Unknown")</label>
</div>
And the fiddle: http://jsfiddle.net/kdUAS/